The present invention relates to an optical communication system, and more particularly to an optical signal quality supervisory device that supervises the quality of a light wave network.
In an optical communication system, the quality supervision of an optical signal is very important for the operation of a network. In a light amplification relay transmission system, the degradation of an optical SNR (Signal-to-Noise Ratio) caused by the degradation of a light amplifier is a factor in the degradation of the quality of an optical signal. A demand for supervising this optical signal with high accuracy has been increased. Also, in a wavelength multiplex system, because a plurality of wavelength channels interfere with each other, the supervision of the optical signal quality with higher precision has been demanded.
In addition, in a coming light wave network, because a plurality of optical network elements (ONEs) constitute a transparent network, if one of those ONEs generates an optical noise, the optical transmission line quality of the entire network is degraded. Thus, a quality supervision high in level is required.
FIG. 16 is a conceptual block diagram showing a light wave network constituted by four ONEs as the optical communication system.
In FIG. 16, reference numeral 90 denotes an optical fiber cable, and 91 to 94 denote optical network elements (ONEs), and each of those ONEs is made up of an optical add-drop multiplexer, an optical cross-correct, an optical line terminal and so on. Reference numeral 95 denotes a main signal transmitter (LINE OS), 96 is a main signal receiver (LINE OR), 97 is a main signal, 98 is an optical supervisory channel transmitter, 99 is an optical supervisory channel receiver that constitutes an optical signal quality supervisory device in association with the supervisory signal optical transmitter 98, and 100 is an optical supervisory channel that is transmitted from the optical supervisory channel transmitter 98 and received by the optical supervisory channel receiver 99.
It is assumed that a trouble such as an increase in the loss of optical parts or a failure of the optical amplifier occurs, for example, in the ONE 92 in FIG. 16.
When the ONE 92 transmits the optical supervisory channel 100, the quality of the optical supervisory channel 100 is degraded by the trouble. The optical supervisory channel receiver 99, upon receiving the optical supervisory channel, detects the degradation of quality and notifies all the ONEs of the trouble through a built-in network management system (NMS).
Incidentally, the conventional optical supervisory channel 100 is made up of a bit interleaved parity (hereinafter referred to as xe2x80x9cBIPxe2x80x9d) byte provided in a section over head (SOH) of a synchronous digital hierarchy (SDH). The BIP byte is made up of a B1 byte (BIP-8) or a B2 byte (BIPNxc3x9724) and counts code errors between the respective relays, between the relay and the line terminal device, or between the respective line terminal devices. The details are disclosed in xe2x80x9cError Rate Degradation Detecting Method in SDHxe2x80x9d Spring Conference of The Institute of Electronics, Information and Communication Engineers, B-762, 1990, written by Fujime et al.
The inner structure of the conventional optical supervisory channel receiver 99 that supervises the quality of transmission line by using the BIP byte is shown in FIG. 17.
In FIG. 17, reference numeral 101 denotes an optical fiber; 102, a photo diode (hereinafter referred to as xe2x80x9cPDxe2x80x9d); 103, a pre-amplifier; 104, a post-amplifier; 105, an equivalent filter; 106, a clock extraction circuit; 107, a discriminator; 108, a serial-parallel conversion circuit; 109, a frame synchronizing circuit; 110, a descrambler circuit; 111, a BIP error detection circuit; 112, a signal degradation (SD) alarm; 113, a section over head (SOH) termination circuit; and 114, a system alarm transfer byte (APS byte).
Subsequently, the operation of the conventional optical supervisory channel receiver 99 will be described.
The optical signal inputted from the optical fiber 101 is photoelectrically converted by the PD 102 and thereafter amplified by the pre-amplifier 103 and the post-amplifier 104. The amplified received signal is subjected to band limit and waveform shaping by the equivalent filter 105. The equivalent filter 105 is normally formed of a quaternary vessel Tomson filter. The equalized signal is branched into two signals, and one of those signals is inputted to the clock extraction circuit 106 from which a clock signal is extracted. The other signal is inputted to the discriminator 107, and then discriminated and reproduced by the extracted clock signal.
The signal discriminated and reproduced by the discriminator 107 is normally developed into 8 parallel signals by the serial-parallel conversion circuit 108, passes through the frame synchronizing circuit 109 and then is descrambled by the descramble circuit 110. Thereafter, the BIP error detection circuit 111 detects an error from the BIP byte separated by the BIP error detection circuit. If the detected error exceeds a preset threshold value, the SD alarm 112 is issued. Also, the APS byte 114 that receives and transmits the supervisory signal between the different ONEs is extracted from the SOH termination circuit 113.
It is assumed that the main signal is, for example, of STM-16 (2.48832 Gbit/s). In this case, the PD 102, the pre-amplifier 103, the post-amplifier 104, the discriminator 107 and the serial-parallel conversion circuit 108 are formed of high-speed semiconductors having a frequency band of 2 GHz or higher. On the other hand, the equivalent filter 105 is set to about 0.7 times the normal bit rate, that is, a frequency band of 1.7 GHz.
However, in the above-described conventional optical signal quality supervisory device, particularly in the optical supervisory channel receiver 99, as the bit rate is higher in speed, it becomes more difficult to constitute the circuit shown in FIG. 17. In particular, the high-speed semiconductor integrated circuit technique is demanded for the clock extraction circuit 106, the discriminator 107 and the serial-parallel conversion circuit 108, accompanied by high costs and increased power consumption. Also, the frame synchronizing circuit 109, the descramble circuit 110 and the BIP error detection circuit 111 increase in circuit scale, and a volume for installing the circuit increases, thereby leading to enlargement of the entire device.
The present invention has been made in order to solve the above-described problems, and therefore an object of the present invention is to provide an optical signal quality supervisory device that is capable of supervising the quality of an optical signal simply, efficiently and with high accuracy without inviting an increase in circuit scale, high costs and an increase in power consumption.
In order to achieve the above object, an optical signal quality supervisory device according to the present invention comprises: optical supervisory channel transmitting means that transmits an optical supervisory channel for supervising the transmission line quality of an optical communication system to a main optical channel receiving means which receives a main optical channel transmitted over the optical communication system, and an optical supervisory channel receiving means that receives the optical supervisory channel transmitted through the optical communication system to supervise the quality of the transmission line, and is characterized in that the bit rate of the optical supervisory channel is made lower than the bit rate of the main optical channel, and in that there are provided, as the optical supervisory channel receiving means, reception discriminating means that receives the signal transmitted through the optical communication system to discriminate and reproduce the optical supervisory channel from the received signal, and error detecting means that has an electric band width narrower than the electric band width of the main optical channel receiving means and detects an error on the basis of the optical supervisory channel that is discriminated and reproduced by the reception discriminating means.
Also, the optical signal quality supervisory device is characterized in that the reception discriminating means has substantially the same electric band width as the electric band width of the main optical channel receiving means.
Further, the optical signal quality supervisory device is characterized in that the reception discriminating means has the electric band width wider than the electric band width of the main optical channel receiving means.
Still further, the optical signal quality supervisory device is characterized in that the discrimination threshold value of the reception discriminating means which is used to discriminate the optical supervisory channel is so set as to be shifted from the optimum threshold value.
Yet still further, the optical signal quality supervisory device is characterized in that another optical supervisory channel receiving means having the electric band width equal to or less than the bit rate of the optical supervisory channel is provided in parallel with the optical supervisory channel receiving means.
Yet still further, the optical signal quality supervisory device is characterized in that the optical communication system comprises a wavelength multiplex system that relays at multiple stages optical amplifiers that amplify a plurality of main optical channels different in wavelength, and in that the optical supervisory channel receiving means includes, at its reception end, a band pass filter a pass band of which is set in accordance with the gain of the optical amplifiers in the wavelength multiplex system.
Yet still further, the optical signal quality supervisory device is characterized in that the pass band of the band pass filter is set to the gain minimum wavelength of the optical amplifiers in the wavelength multiplex system.
Yet still further, the optical signal quality supervisory device is characterized in that the pass band of the band pass filter is set to the gain peak wavelength of the optical amplifiers in the wavelength multiplex system.
Yet still further, the optical signal quality supervisory device is characterized in that the optical supervisory channel transmitting means includes sweeping means for discretely sweeping the wavelength of the optical supervisory channel between the wavelengths of the adjacent main optical channels.
Yet still further, the optical signal quality supervisory device is characterized in that the sweeping means comprises: a light source that generates a noise light over a wide band; a supervisory signal source; a wavelength selection filter connected to the light source and swept by a step-like signal generated from a wavelength sweep signal synchronous with the supervisory signal source; a modulator that modulates the noise light from the light source which is cut off by the wavelength selection filter according to the supervisory signal source; and an optical shutter that shuts out an output of the modulator when crossing the wavelength of the main signal.
Yet still further, the optical signal quality supervisory device is characterized in that the wavelength selection filter is formed by connecting in series Fabry-Pe rot etalon a periodic transmission peak of which is set to the center of the wavelength multiplex intervals of the main optical channel to a tunable filter having a transmission characteristic that is sharp in narrow band.
Yet still further, the optical signal quality supervisory device is characterized in that the supervisory optical channel has a signal format of a synchronous digital hierarchy, and in that the error detecting means conducts the quality supervision of the optical signal by the error detection by a bit interleaved parity of a section over head.
Yet still further, the optical signal quality supervisory device is characterized in that the supervisory optical channel has a forward error correction, and in that the error detecting means conducts the quality supervision of the optical signal by the error detection when the correction code is decoded.